Method of plasma etching with parallel plate reactor having a grid

ABSTRACT

A parallel plate reactor having a grounded grid disposed between an RF powered electrode and a grounded electrode upon which a substrate is disposed. A method of utilizing the above apparatus consists of etching the substrate using a composition of 30-100% NF3 (nitrogen trifluoride) at 25 SCCM (standard cubic centimeter per minute) and 0-70% He (helium) at 75 SCCM to etch a layer of PECVD (plasma enchanced chemcial vapor deposition) Si3N4 (silicon nitride). The etching takes place at 200 mtorr to 5 torr pressure and 50-400 watts RF power.

This application is a continuation of application Ser. No. 07/238,462,filed Aug. 30, 1988 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to reactors used insemiconductor processing and, more particularly, to parallel platereactors and their method of use.

Typically, semiconductor devices are manufactured by using combinationsof differing layers of conductive, insulating, and semiconductingmaterials. It is often necessary to form openings in an upper substratelayer so that contact can be made with an underlying substrate layer. Toaccomplish this, a mask is deposited on the substrate. The mask ispatterned to cover selected regions while leaving other regions exposed.The wafer is then subjected to a corrosive type of environment whichwill etch the exposed portion of the substrate. However, the etchmaterials also tend to etch the masks. It has been an ongoing objectiveof the industry to develop etching methods and apparatus which willselectively etch substrates. In other words, methods that will etch thesubstrate at a rate faster than the mask.

By increasing the differences in etch rates between the substrate andmask, the etching can be faster and permit the use of thinner masks.This provides an economic saving to the manufacturer. One type ofselective etching is described in U.S. Pat. No. 4,568,410 invented byStephen C. Thornquist.

Etchers come in two basic types, dry and wet. Wet etchers use a liquidelement in which the substrates are submerged in order to etch thesubstrate. This type of etching is very traumatic and can cause damageto the substrates. Dry etchers use gas which forms a plasma to etch thesubstrate. Dry etching techniques are often referred to as reactive ionetching or plasma etching. In the prior art types of dry etchers, aplasma is formed by injecting a gas into an area between two electrodes.The reaction of the gas to the RF field produces a plasma whichgenerates ions. The ions then collide with the substrates causingportions of the surface of the substrate to etch, or chip, off.

There are three basic types of reactors that use dry type etching toperform the etching function: barrel reactors, downstream reactors, andparallel plate reactors. The barrel and parallel plate type of reactorsetch by generating a plasma envelope (or sheath). The ions generated inthis plasma then bombard the substrate which is at a different,generally lower, potential than the electrodes. A disadvantage of thesetypes of etchers is that the substrate is subjected to a much moreintense ion bombardment because of the difference in potential betweenthe plasma and the substrate. This makes it much more difficult to varythe etch rate between mask and substrate.

In the downstream reactors, the plasma is generated remotely from thesubstrate and then by normal diffusion reaches the substrate. A majorhistorical disadvantage of these types of reactors is that they havebeen made of quartz which is more expensive and more subject to damagethan the metal and ceramic type of reactors. Quartz is consumed, oretched away, in a fluorine based etch. This not only damages the chamberbut can also leave residue on the substrate.

Accordingly, it is an object of the present invention to provide animproved apparatus and method that will overcome the above noteddeficiencies.

A further object of the present invention is to provide a parallel platereactor and method that improves the selectivity of etch ratios.

Another object of the present invention is to provide a parallel platereactor which provides the advantages of wet etching in a dry etchenvironment.

Still another object of the present invention is to provide a parallelplate reactor and method that improves the etch rate.

Yet another object of the present invention is to provide a parallelplate reactor and method that requires a lower ratio of mask tosubstrate thickness.

Another object of the present invention is to provide a parallel platereactor and method which improves the selective etching of a fluorinegas sources.

Still another object of the present invention is to provide a parallelplate reactor and method which will selectively etch low pressurechemical vapor deposition (LPCVD) nitride (Si₃ N₄).

Yet another object of the present invention is to provide a parallelplate reactor and method which will etch phosphorous or boron dopedoxide, isotropically.

SUMMARY OF THE INVENTION

A particular, preferred embodiment of the present invention consists ofa parallel plate plasma reactor having a grounded grid disposed betweenan RF powered electrode and a grounded electrode upon which a substrateis disposed. The above apparatus is utilized in one particular gascomposition of 30-100% NF₃ (nitrogen fluoride) at 25 SCCM (standardcubic centimeter per minute) and 0-70% He (helium) at 75 SCCM to etch alayer of PECVD (plasma enhanced chemical vapor deposition) Si₃ N₄(silicon nitride). The etching takes place at 200 mtorr to 5 torrpressure and 50-400 watts RF power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross sectional view of a prior art barrelreactor;

FIG. 2 is a simplified cross sectional view of a prior art downstreamreactor;

FIG. 3 is a simplified cross sectional view of a prior art parallelplate reactor;

FIG. 4 is a simplified cross sectional view of a second prior artparallel plate reactor; and

FIG. 5 is a cross sectional view of a parallel plate reactor embodyingthe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a simplified cross sectional view of a prior artbarrel reactor, generally designated 10, is illustrated. Barrel reactor10 consists of a quartz cylinder 11 having a cage 12 and an electrode13. Electrode 13 is coupled to a power supply 14 which provides a 13.56MHz signal. Cage 12 and electrode 13 are spaced apart by a pair ofinsulator legs 15. A substrate 16 is disposed within an area 17 definedby cage 12.

When operating, cage 12 assumes an electrical charge that reduces theion flux experienced by substrate 16. A plasma envelope, whose perimeteris represented by dotted line 18, is generated within cylinder 11.

Because of the difference in potential between the plasma and substrate16, and between cage 12 and substrate 16, ions bombard substrate 16atomically chipping away at the surface. A drawback of barrel reactor 10is that it is more difficult to selectively etch the substrate layer andmask where this type of chipping occurs.

In FIG. 2, a simplified cross sectional view of a downstream type ofreactor, generally designated 20, is shown. Reactor 20 consistsessentially of a power supply 21 and a chamber, generally quartz, 22coupled together by a pipe 23, also generally made of quartz. In etcher20, a substrate 24 is disposed on an insulator pedestal 25.

In operation, etcher 20 generates a plasma envelope 26 in pipe 23. Theions and neutrals generated diffuse along pipe 23 to chamber 22 wherethey etch substrate 24. While plasma source 26 is removed from substrate24, there is no guarantee that plasma 26 and it's electrical potentialare electrically confined near power source 21.

A third type of reactor is a parallel plate reactor. A simplified crosssectional view of a prior art parallel plate reactor, generallydesignated 30, is shown in FIG. 3. Reactor 30 consists essentially of anupper electrode 31 and a lower electrode 32 having a substrate 33disposed therebetween. Upper electrode 31 is coupled to a housing 34 toform a chamber 38. An insulator 35 is mounted on the base of housing 34.On insulator 35 is mounted lower electrode 32. Substrate 33 then restson lower electrode 32.

In operation, lower electrode 32 is coupled to a power supply 36 andupper electrode 31 is coupled to ground. A plasma envelope, designatedby dotted line 37, is generated that engulfs substrate 33. Thedifference in potentials between the plasma and substrate 33 causessubstrate 33 to be subjected to high energy bombardment by the ions.This atomic chipping action provides less selectivity in etching than isprovided by chemical etching.

Another prior art parallel plate reactor is illustrated in FIG. 4. Thissecond reactor has been designated as 30' with items similar to reactor30 of FIG. 3 being designated in the same manner with the addition of aprime ('). A grid 39 has been disposed in chamber 38' of reactor 30'.Grid 39 is grounded while upper and lower electrodes, 31' and 32'respectively, are powered. This places substrate 33' in a situationwhere it still has an electrical potential different from that ofelectrodes 31' and 32'. Having a different potential results in theatomic chipping type of etch which is less selective.

Referring now to FIG. 5, a cross sectional view of a parallel platereactor, generally designated 40, embodying the present invention isillustrated. Reactor 40 consists primarily of an upper electrode portion41, an upper RF chamber 42, a lower chamber 43, a lower electrode 44,and a grid 45. As illustrated in FIG. 4, a plasma envelope 46 isgenerated between grid 45 and electrode 41.

Electrode 41 consists of a cap 47 having a gas opening 48 therein. Cap48 is mounted on a lid 49. Lid 49 also contains a gas opening 50 whichcoincides with opening 48 of cap 47. As is known in the art, lid 49 cancontain vias for the circulation of water. The circulating water is usedto cool reactor 40. Cap 47 acts as a cover for the water vias.

Lid 50 is disposed on a bottom disc 51 to form a gas mixing chamber 52therebetween. An O'ring is disposed in an opening 53 formed between lid49 and disc 51. Disc 51 contains a plurality of openings 54 throughwhich the reactant gases pass once mixed. The reactant gases passthrough openings 54 into upper RF chamber 42 where plasma 46 is formed.The walls of chamber 42 are provided by an insulator 55. Insulator 55 ispreferably made of aluminum oxide (Al₂ O₃). An opening 56 is formedbetween disc 51 and insulator 55 in which a second O'ring is disposed.

Defining the bottom of chamber 42 is grid 45. Grid 45 is an electricallyconductive perforated aluminum sheet type of grid, but may be made ofmany differing shapes. Grid 45 is coupled to the inner surface of body57. Disposed about grid 45 and defining the circumference of lowerchamber 43 is a chamber body 57. At the interface between chamber body57 and insulator 55 is an O'ring opening 58.

Chamber body 57 is mounted on a base plate 59. At the interface betweenchamber body 57 and base plate 59 is an O'ring opening 60. Mounted onbase plate 59, inside lower chamber 43, is an insulator 61. Insulator 61is comprised of Aluminum Oxide (Al₂ O₃). It should be noted here thatsince grid 45 and lower electrode 44 are at the same potential,insulator 61 is optional. Mounted on insulator 61 is lower electrode 44.Also mounted on insulator 61, disposed about the circumference of lowerelectrode 44, is a vacuum baffle 62. Vacuum baffle 62 forms an exhaustchamber 63 through which the gases in chamber 43 are removed.

A substrate 64, such as a semiconductor wafer, is disposed on thesurface of lower electrode 44 such that a surface of substrate 64 isfacing lower chamber 43.

In operation, reactant gases flow through openings 48 and 50 intochamber 52 where they are mixed. Typical gasses utilized consist of afluorine (F) source gas such as NF₃, CF₄, or SF₆. The gases are thenmixed in chamber 52 and passed through openings 54 into upper RF chamber42. While in chamber 42, the gases are disposed between upper electrode41, which is coupled to an RF power supply 65, and grid 45, which isgrounded. This excites the gases forming plasma envelope 46. Ions formedby the plasma then migrate through grid 45 into chamber 43.

A key to the present invention is that grid 45 and lower electrode 44are at the same potential, grounded in this embodiment. In the prior artof FIG. 4, grid 45 and lower electrode 44 are at different potentials.The prior art utilizes this to attract the ions toward substrate 64which causes an atomic chipping type etching when the ions impact thesubstrate. In the present invention, by having the lower electrode andthe grid at the same potentials, the acceleration of the ions toward thesubstrate is removed. The present invention provides more of a chemicalreaction type of etching. This chemical type of etching results in amuch more selective etch between the mask and substrate layer.

Tests were run to demonstrate the enhanced selectivity of the presentprocess. One test consisted of the etching of silicon nitride (Si₃ N₄)deposited on a substrate using plasma enhanced chemical vapor deposition(PECVD). The process conditions were:

    ______________________________________                                        Pressure:      600 mtorr                                                      Power:         175 watts RF (13.56 mHz)                                       Gases:         NF.sub.3 at 75 Sccm's                                                         He at 75 Sccm's                                                ______________________________________                                    

The resulting etch rates (in Angstroms/minute) are listed below:

    ______________________________________                                        MATERIAL     WITHOUT GRID  WITH GRID                                          ______________________________________                                        Etch Rates:                                                                   PECVD (Si.sub.3 N.sub.4)                                                                   >9000 A/min   >9000 A/min                                        Photoresist  >1000 A/min   <1000 A/min                                        Selectivity:                                                                  PECVD: Photoresist                                                                         9:1           9:1                                                ______________________________________                                    

In the above process, it was found that: the power can be varied from 50to 400 Watts; the pressure can range between 200 millitorr and 5 torr;and the percentage of NF₃ in He ranged from 30% to 100%.

In a second test, an etch was performed on low pressure chemical vapordeposition (LPCVD) silicon nitride under the following conditions:

    ______________________________________                                        Pressure:       500 mtorr                                                     Power:          50 watts RF (13.56 mHz)                                       Gases:          NF.sub.3 at 25 Sccm's                                                         O.sub.2 at 50 Sccm's                                          Chuck Temp:     60° C.                                                 ______________________________________                                    

Since this process has not been performed in a standard parallel plateetcher, only the results for a parallel plate etcher with grid groundedto the lower electrode are given. The results were:

    ______________________________________                                        MATERIAL         WITH GRID                                                    ______________________________________                                        Etch Rates:                                                                   LPCVD (Si.sub.3 N.sub.4)                                                                       >1500 A/min                                                  Photoresist        200 A/min                                                  Thermal Oxide      150 A/min                                                  Selectivity:                                                                  LPCVD: Photoresist                                                                             >7.5:1                                                       LPCVD: Oxide      10.0:1                                                      ______________________________________                                    

In this process: the power can vary between 50 to 150 watts; thepressure can vary between 200 millitorr and 5 torr; and the gas mixesrange from 20-80% of NF₃ in O₂.

A third process tested was that of etching isotropic, phosphorous andboron doped, oxide under the following conditions:

    ______________________________________                                        Pressure:      1.5 torr                                                       Power:         300 watts RF (13.56 mHz)                                       Gases:         NF.sub.3 at 50 Sccm's                                                         He at 50 Sccm's                                                ______________________________________                                    

Again, it is not possible to perform this process in a standard parallelplate reactor. Therefore, only the results are provided for a reactor ofthe type of the present invention. The results were:

    ______________________________________                                        MATERIAL        WITH GRID                                                     ______________________________________                                        Etch Rates:                                                                   Oxide           >2500 A/min                                                   Photoresist       250 A/min                                                   Selectivity:                                                                  Oxide: Photoresist                                                                            >10:1                                                         ______________________________________                                    

In this process, the applicable process ranges are the same as the firstprocess.

Thus, following a review of this specification, it will be apparent toone skilled in the art that there has been provided in accordance withthe invention, a device and method that fully satisfies the objects,aims, and advantages set forth above.

While the invention has been described in conjunction with specificembodiments thereof, it will be evident that many alterations,modifications, and variations will be apparent to those skilled in theart in light of the forgoing description. Accordingly, it is intended toembrace all such alterations, modifications, and variations in theappended claims.

I claim:
 1. A method of etching a substrate comprising the stepsof:placing the substrate on a surface in an apparatus comprising anupper electrode coupled to an R-F power source, a lower electrode and agrid having openings defined therein and disposed between said upperelectrode and said lower electrode, said grid and upper electrodeforming an upper chamber therebetween, said grid being clamped to havethe same potential at all times as the lower electrode and forming withthe lower electrode a lower chamber therebetween; passing a reactant gasinto said upper chamber; subjecting said gas to an R-F field applied viasaid upper electrode to generate in said upper chamber a plasmaincluding ions in said upper chamber; passing ions generated by saidplasma through said grid toward said lower chamber; and etching saidsubstrate on said surface, said surface being in said lower chamber,said substrate being etched by said ions.
 2. The method of claim 1wherein said reactant gas comprises a fluorine source gas.
 3. The methodof claim 2 wherein said fluorine source gas comprises nitrogentrifluoride.
 4. The method of claim 2 wherein said fluorine source gascomprises carbon tetraflouride.
 5. The method of claim 2 wherein saidfluorine source gas comprises sulfur hexafluoride.
 6. The method ofclaim 1 wherein the substrate comprises a semiconductor device.
 7. Themethod of claim 6 wherein said reactant gas comprises a fluorine sourcegas.
 8. The method of claim 7 wherein said fluorine source gas comprisesnitrogen trifluoride.
 9. The method of claim 7 wherein said fluorinesource gas comprises carbon tetraflouride.
 10. The method of claim 7wherein said fluorine source gas comprises sulfur hexafluoride.
 11. Themethod of claim 1 wherein said substrate includes a layer of siliconnitride and the reactant gas is nitrogen trifluoride.
 12. The method ofclaim 11 in which said upper electrode is coupled to a power source inthe range of approximately 50-400 watts.
 13. The method of claim 12wherein said power source is approximately 175 watts.
 14. The method ofclaim 11 in which the nitrogen trifluoride concentration is in the ofapproximately 30-100% with the remainder being helium.
 15. The method ofclaim 14 wherein the nitrogen, trifluoride concentration isapproximately 50% with the remainder being helium and said helium issupplied at a rate of approximately 75 standard cubic centimeters perminute.
 16. The method of claim 11 wherein said etching occurs in therange of 200 millitorr to 5 torr pressure.
 17. The method of claim 16wherein said etching occurs at approximately 600 millitorr.
 18. Themethod of claim 1 wherein the substrate includes a layer comprising alow pressure chemical vapor deposition of silicon nitride.
 19. Themethod of claim 18 in which said upper electrode is coupled to a powersource in the range of approximately 50-150 watts.
 20. The method ofclaim 19 wherein said power source is approximately 50 watts.
 21. Themethod of claim 18 in which the nitrogen trifluoride concentration is inthe range of approximately 20-80% with the remainder being oxygen. 22.The method of claim 21 wherein the nitrogen trifluoride concentration isapproximately 67% with the remainder being oxygen and said oxygen issupplied at a rate of approximately 25 standard cubic centimeters perminute.
 23. The method of claim 18 wherein said etching occurs in therange of 200 millitorr to 5 torr pressure.
 24. The method of claim 23wherein said etching occurs at approximately 500 millitorr.
 25. Themethod of claim 18 wherein said lower electrode is cooled toapproximately 60° C.
 26. The method of claim 1 wherein the substrateincludes a layer of an impurity doped oxide and the reactant gas isnitrogen trifluoride.
 27. The method of claim 26 wherein said impuritydoped oxide comprises phosphorous doped oxide.
 28. The method of claim26 wherein said impurity doped oxide comprises boron doped oxide. 29.The method of claim 26 in which said upper electrode is coupled to apower source in the range of approximately 50-400 watts.
 30. The methodof claim 29 wherein said power source is approximately 300 watts. 31.The method of claim 26 in which the nitrogen trifluoride concentrationis in the range of approximately 30-100% with the remainder beinghelium.
 32. The method of claim 31 wherein the nitrogen trifluorideconcentration is approximately 50% with the remainder being helium andsaid helium is supplied at a rate of approximately 50 standard cubiccentimeters per minute.
 33. The method of claim 26 wherein said etchingoccurs in the range of 200 millitorr to 5 torr pressure.
 34. The methodof claim 33 wherein said etching occurs at approximately 1.5 torr.